Tree structured multicarrier multiple access systems

ABSTRACT

A Tree-structured multicarrier multiple access communication system in a multiple access environment having a number of interfering signals. In one embodiment an environment estimator estimates and tracks the number of interfering signals of the environment. A code selector determines an optimal spreading code represented by the environment, and identifies an actual spreading code that is closest to the optimal spreading code. A signal transmitter can then be used to transmit a multicarrier multiple access signals in accordance with the actual spreading code identified. On the receiver side, a multiuser detector (MUD) module performs tree structured multiuser detection signal processing, and produces a bit stream for each interfering signal.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/632,098, filed Dec. 1, 2004 which is herein incorporated in itsentirety by reference.

FIELD OF THE INVENTION

The invention relates to communication systems, and more particularly,to multicarrier systems with multiuser detection.

BACKGROUND OF THE INVENTION

There has been considerable attention on using multicarriercommunication techniques to transmit information over wireless channels.For example, orthogonal frequency division multiplexing (OFDM) has beenpicked as the basis for the 802.11a and 802.11g standards for wirelesslocal area networks.

Multicarrier systems have several features which make them attractive tothe wireless medium. For one thing, the equalization task in a welldesigned system can be simply implemented as a multiplication on eachsubcarrier rather than via a RAKE-receiver scheme or a convolution.Moreover, estimating the parameters required for receiver processing isreliably done with a simple algorithm since the signals are disjoint andnarrow band, each experiencing a simple flat fading.

The multicarrier system can also be set up so that some or all the usershave orthogonal waveforms. Multicarrier systems can conceptuallymaximize the information capacity of the network by careful allocationof power and bit rate among its subcarriers.

However, the multicarrier systems present significant challenges to thesystem designer. Several schemes have been proposed to meet thesechallenges, many of which use pilots or training sequences.

Conventional methods of wireless “multiple access” communication havedifficulty with interference when many devices are assigned to a singlewireless network, as well as when another network of wireless devicesexists nearby at the same frequency. When there is interference, suchconventional systems may begin to output a higher bit error rate (BER)and, may possibly fail to operate reliably, thus dropping devices andconnections in a seemingly random fashion.

Tree structured interference is known to allow for an overpacking of theavailable channel resources, while allowing for an optimal multiuserjoint detector to be implemented in real-time. In contrast, optimaljoint detection for interfering (non-orthogonal) users in a multipleaccess communication system generally has a computational complexitythat increases exponentially with the number of users. Thus, practicallimitations necessitate the integration of tree structured techniquesinto multiple access systems such as wireless local area networks,mobile and cellular terrestrial systems, and satellite-based systems.

There is a need, therefore, for techniques that alleviate degradedperformance and throughput in a wireless communication system due tointerference from other transmitters in the system assigned to eitherthe same network or to a different nearby network.

SUMMARY OF THE INVENTION

One embodiment of the present invention provides a multicarrier systemconjoined with tree structured multiple access schemes to take advantageof the benefits provided by both multicarrier systems and multiuserdetection (MUD) schemes. One embodiment is a Tree Structured orthogonalfrequency division multiple access (TS-OFDMA) and another embodiment isa Tree Structured Multicarrier Code Division Multiple Access(TS-MC-CDMA).

Another embodiment of the invention is a tree-structured multicarrierinterference multiple access (MC-IMA) transceiver for use in a multipleaccess environment having a number of interfering signals, comprising anenvironment estimator module configured to estimate and track the numberof interfering signals of the environment. There is a multicarriersignature signal selector configured to determine a multicarriersignature signal as a function of the environment, and to identify anactual multicarrier signature signal that is closest to the multicarriersignature signal. A signal transmitter is configured to transmit amulticarrier multiple-access signal in accordance with the actualmulticarrier signature signal identified by the multicarrier signaturesignal selector. A multiuser detector (MUD) module is configured toperform tree structured multiuser detection signal processing, and toproduce a bit stream for each interfering received signal.

Other aspects of the invention includes wherein the multicarriersignature signal selector is configured to determine an optimalmulticarrier signature signal. The multicarrier signature signal canalso be any multicarrier signature signal that abides by thetree-structure.

Another aspect includes the transceiver further comprising a stream ofinterest selector configured to select one or more bit streams ofinterest.

A feature of the invention includes wherein the multicarrier signaturesignal selector is further configured to determine received powers andrates of each active user in the environment, and to determine a channeldegradation value for a communication link between the transceiver and adestination receiver.

Further aspects of the transceiver include wherein the multicarriersignature signal selector is further configured to build a set of parentmulticarrier signature signals based on a known set of basis subcarriersused in the base set of multicarrier signature signals, and to build aset of grandparent multicarrier signature signals based on either theset of parent multicarrier signature signals or the basis set ofsubcarriers from which those signals were comprised, and to continuecreating ancestor nodes in the same manner until there are no more nodesto combine in this manner. An optional feature includes deleting any ofthe nodes and corresponding signature signals from the set oftree-structured multicarrier signature signals.

The code selector can be further configured to arbitrarily makingsibling groups of one or more base multicarrier signature signals pergroup, not allowing any one base multicarrier signature signal to have anon-zero or non-negligible cross correlation with any signal in morethan one group, and creating a new multicarrier signature signal bylinearly combining all the base spreading codes within a sibling groupor by constructing a new multicarrier signature signal from the basisset of subcarriers present in the aggregate of all the signals within asibling group.

The transceiver wherein the code selector is further configured toarbitrarily making sibling groups of one or more parent multicarriersignature signals per group, not allowing any one multicarrier signaturesignal to have a non-zero or non-negligible cross correlation withmulticarrier signature signals in more than one group, and linearlycombining all the multicarrier signature signals or creating a newmulticarrier signature signal from the set of subcarriers used within asibling group.

A method embodiment includes performing tree-structured multicarrierinterference multiple access (MC-IMA) communication in a multiple accessenvironment having a number of interfering signals, comprisingestimating and tracking the number of interfering signals of theenvironment, determining a multicarrier signature signal represented bythe environment, and identifying an actual multicarrier signature signalthat is closest to the multicarrier signature signal, transmitting amulticarrier multiple access signal in accordance with the actualmulticarrier signature signal, and performing tree structured multiuserdetection signal processing, and producing a bit stream for eachinterfering signal.

The multicarrier signature signal can be a multicarrier signature signalthat abides by the tree-structure an optimal or an optimal multicarriersignature signal. Determining the optimal multicarrier signature signalmay further include determining received powers and rates of each activeuser in the environment, and determining a channel degradation value fora communication link between the transceiver and a destination receiver.

The method may include determining a multicarrier signature signalcomprises building a set of parent multicarrier signature signals basedon a known set of base multicarrier signature signals or the set ofsubcarriers used by the base set of users, building a set of grandparentmulticarrier signature signals based on the set of parent multicarriersignature signals or the set of subcarriers used by the base set ofusers, and continuing this progression upward by doing the same for aset of great-grandparent multicarrier signature signals and so on, untilall nodes have been linked. This may further include deleting any of thenodes and corresponding signature signals from the set oftree-structured multicarrier signature signals.

The method may further include building a set of parent multicarriersignature signals comprising arbitrarily making sibling groups of one ormore base multicarrier signature signals per group or set of subcarriersused by that group, not allowing any one base multicarrier signaturesignals to have a non-zero cross correlation with signals in more thanone group, and creating a new multicarrier signature signal by linearlycombining all the base spreading codes within a sibling group orconstructing a new multicarrier signature signal from the basis set ofsubcarriers present in the aggregate of all the signals within a siblinggroup.

The method may further include building a set of grandparentmulticarrier signature signals comprising arbitrarily making siblinggroups of one or more parent multicarrier signature signals or set ofsubcarriers used by a group of parents, not allowing any one parentmulticarrier signature signals to have a non-zero non-negligible crosscorrelation with any multicarrier signature signal in more than onegroup, and creating a new multicarrier signature signal by linearlycombining all the base spreading codes within a sibling group orconstructing a new multicarrier signature signal from the basis set ofsubcarriers present in the aggregate of all the signals within a siblinggroup.

The features and advantages described herein are not all-inclusive and,in particular, many additional features and advantages will be apparentto one of ordinary skill in the art in view of the drawings,specification, and claims. Moreover, it should be noted that thelanguage used in the specification has been principally selected forreadability and instructional purposes, and not to limit the scope ofthe inventive subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a prior art single user multicarrier system wherein an OFDMuser's spectrum is transmitting on 16 carriers.

FIG. 1 b shows a prior art OFDMA system shown with two users, whereineach user is assigned a subset of the subcarriers to send itsinformation.

FIGS. 2 a-2 d shows four users in an MC-CDMA system, wherein each usersends the carriers with the relationship detailed herein, and thesubcarriers are all modulated by the same information symbol butdifferent elements of the spreading code.

FIG. 3 is a diagrammatic perspective of a broadcast channel embodimentwherein the satellite can control each of the users' signature waveformsto adhere to the tree structure and provide the necessary information toeach terminal, and wherein each terminal can then perform the tree-MUDreceiver algorithm to extract the signal associated with its branch ofthe tree.

FIG. 4 illustrates tree-structured MC-CDMA transceiver configured inaccordance with one embodiment of the present invention.

FIG. 5 a illustrates a tree structure embodiment wherein each of thesignals associated with any one bottom node is orthogonal to all othersignals associated with all other bottom nodes, and any single signalassociated with a single node on the next level is orthogonal to allother signals associated with all other nodes at that same level buthave correlation to signals associated with any nodes it is connected toat the lower or upper level, including upper or lower levels thatrequire traversing through any number of upper or lower level nodes toreach.

FIG. 5 b illustrates an example correlation tree for any generic set of11 user signatures.

FIGS. 6 a-d shows the 16 signals for the base nodes of the treestructure in a tree-packed MC-CDMA system, configured in accordance withone embodiment of the invention.

FIG. 7 shows a 5th user added to each group to make the systemoverloaded, wherein this figure shows four signature signals, one foreach group, and wherein these four signature signals are orthogonal toone another because they use disjoint sets of subcarriers.

FIG. 8 shows 2 user signature signals, corresponding to users c1 and n1in the tree, wherein each user's signature signal is built from 8subcarriers each and the link between these signature signals and theircorrelated sets upon which they were built is detailed herein.

FIG. 9 shows the top node in the MC-CDMA example wherein the user iscorrelated with all other users in the system, and uses all thesubcarriers.

FIG. 10 is a detailed tree structure for the MC-CDMA example, configuredin accordance with one embodiment of the invention.

FIG. 11 Block diagram of a wireless multicarrier system with a treestructured multiuser detector, wherein each user could be an OFDMAtransmitter communicating with a base station in a cellular system,configured in accordance with one embodiment of the invention.

FIG. 12 is a perspective view of an overloaded OFDMA example treestructure, configured in accordance with one embodiment of theinvention.

FIG. 13 illustrates a method for creating tree-structured MC-CDMAsignature signals to allow for overpacked MC-CDMA communication inaccordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Multi-carrier code division multiple access (MC-CDMA) systems can beused, for example, for mobile cellular communications and other wirelessnetworks. MC-CDMA can generally be described as a combination oforthogonal frequency division multiplexing (OFDM) and CDMA. Stated inanother manner, OFDM is a subset of MC-CDMA. CDMA was designed for thepurpose of allowing multiple users to occupy the same frequency channelsimultaneously, and assigns individual users a unique spreading codethat is orthogonal to all other user's spreading codes. When spread,individual users appear as noise to all other users. OFDM, which wasdesigned for the purpose of reliably transmitting data overtime-dispersive or frequency selective environments without the use ofcomplex time-domain channel equalizers, takes advantage of theproperties of the Fast-Fourier Transform (FFT) to provide orthogonalsub-channels. In particular, OFDM takes a large high-rate bandwidth anddivides it into overlapping orthogonal lower-rate sub-channels, andoffers the advantages of ease of channel equalization and the ability toallow variable data rates per sub-channel. Since the transmittedinformation is broken into several narrowband channels, it ensures flatfading over each sub-channel, allowing for equalization with a complexdivide at the receiver. Using knowledge of the channel and the type ofservice being requested, the selection of the waveform at eachsub-channel can be dynamically adapted to maximize the informationcarrying capacity of the sub-channel.

Demand for channel use can be responded to, for example, even when itrequires assigning more code signatures or channels than can co-existorthogonally. There are numerous ways of re-assigning the bandwidth. Forexample, overlapping frequency division multiple access (FDMA) bands ortime division multiple access (TDMA) slots can be used to create a sortof “domino” packing where adjacent users interfere heavily. In a directsequence spread spectrum (DSSS) system users may take on any Walsh code,regardless of whether or not it has already been assigned. Bandwidth isdirectly re-assigned such that users interfere with each other in thesame manner a spoof signal interferes (e.g., “right on top” of eachother). Users are assigned waveforms that interfere in a tree-structuredway, as will be explained herein.

The loading of a wireless communication systems is generally describedas the number of users per the number of dimensions in the transmitspace. Overloading is caused by the proliferation of networks withoutexpanding the set of resources. As previously stated, performanceimpacts due to interfering signals during an overloaded conditioninclude a higher BER and an unreliable communication link. Mosttraditional communication systems treat multiuser interference as noise,which causes severe degradation of all individual users' receiverperformance. Instead of treating interference as noise, an embodiment ofthe present invention uses the specifications of the interfering signalsand cancels them.

In accordance with an embodiment of the present invention, each MC-CDMAradio (also referred to as a transceiver) of the wireless communicationsystem knows the specifications of the interfering signals. Forinstance, each radio knows the dictionary of signature waveforms and/or“spreading” codes, possible operating frequencies, baud rates,data/voice compression rates, dictionary of regularly placed orseemingly randomly placed known bits that act as training sequences ordata-punctured pilots, error correction codes, and possibly encryptionkeys.

Each radio's receiver is always performing parameterestimation/tracking, so it always knows how many users are active, thecode or signature waveform of each active user, the received power,carrier phase, channel (multipath), Doppler, timing offset, and otherpertinent system parameters. Each radio is configured to performmultiuser detection in addition to the parameter estimation module. Insome embodiments, the MUD module might be part of the parameterestimation module. In any case, degraded performance and throughput inthe wireless communication system due to interference from othertransmitters in the system assigned to either the same network or to adifferent nearby network is reduced.

The present invention can be used in various implementations. Forexample, a controller based system employs a controller that assignseach user with a signature waveform that conforms to the tree structure.An example would be a cellular system that requires a handshake before auser becomes active in the system. This handshake would typically assignthe user its set of subcarriers and code. Each user can be communicatingwith any subset of other users as long as the tree structure ismaintained.

Referring to FIG. 1 a, a basic single user multicarrier system is shown,wherein an OFDM user's spectrum is transmitting on 16 carriers. Eachsubcarrier has a different information symbol modulated on it (theactual sequence is [−1 1 −1 −1 . . . 1 1 −1]). The subcarrier's spectrumis illustrated as half sine waves for simplicity, and in someembodiments they would be sinc functions. A user is sending informationon 16 carriers and is modulating each subcarrier with a separate datasymbol. The distance between each subcarrier in the frequency domain isselected so that each carrier appears to have gone through a flat fadingchannel and thus can be easily estimated and equalized by multiplying bythe inverse of the gain induced by the channel.

Referring to FIG. 1 b, to make a multicarrier transmitter cooperate in amultiple-access environment a special structure is generally assigned toeach user in the system. This can be done in several ways and a fewexamples are illustrated to explain the conceptual framework. First,each user can be assigned a subset of subcarriers from a large group ofsubcarriers.

The example depicted in FIG. 1 b shows two users sharing the spectrumusing separate subcarriers. This is referred to as orthogonal frequencydivision multiple access (OFDMA). The Users are designated as follows:

r User;

b User.

In this scenario it is usually assumed that each user is assigned asubcarrier and no other user can use that subcarrier. The presentinvention is not restricted to the disjoint frequency case, instead, itallows users to share subcarriers to provide for a larger number ofusers in the system. Current systems using OFDMA include 802.16e,sometimes known as Mobile WiMAX.

Another option for multiple access using multicarrier systems ismulticarrier CDMA (MC-CDMA). This is the approach of assigning spreadingcodes to the subcarriers used by a single user. For example, a user maysend a single bit over many subcarriers with the polarity of eachsubcarrier being a function of the bit and the chip (element of thespreading code) that is assigned to that subcarrier. FIGS. 2 a-2 d showsthis system with four users sharing four subcarriers. The first subplotshows the carrier orientation of User 1. User 1 adjusts the polarity ofevery carrier based on the polarity of its information bit. Therefore,its spectrum is shown with an ambiguity in the polarity due to themodulation of the symbol. The same technique is used for the other usersbut they have different carrier orientations (specified by the spreadingcode).

The MC-CDMA approach has many of the same advantages of direct-sequenceCDMA (DS-CDMA) for narrowband interference suppression and a gracefuldegradation of system performance (bit error rate) as the number ofusers increases. See, for example, R. V. Nee and R. Prasad, “OFDM forWireless Multimedia Communications”; Boston, Mass., Artech House, 2000.Users are separated at the receiver by the assigned spreading codes. Intypical systems, the number of users is limited by the length of thespreading code because users are designed to be orthogonal (or nearlyorthogonal). This type of system generally does not permit overloading,and if more users are allowed to transmit in the system, thenorthogonality cannot be maintained and complex receiver structures mustbe employed.

Method For Overloading an OFDMA or MC-CDMA

When the number of users in any of the proposed multiple-accessmulticarrier systems increases past the number of degrees of freedom inthe system, it becomes difficult or impossible to guarantee that theusers' signals are separable by conventional means (orthogonal). In thiscase, a multiuser detector can be employed to significantly improve thesystem performance. Multiuser detectors try to make decisions about whatevery user in the system is sending instead of treating one user as thedesired user and the other users as noise. One of the current limitingfactors in the application of MUD to wireless communications is thecomplexity involved in solving for the joint estimate of all users'information, as well as the current lack of good parameter estimationalgorithms to produce the required knowledge of the received signalparameters to the MUD. To mitigate the complexity issue of MUD, signalpacking schemes have been proposed which can decrease the amount ofprocessing that must be done to find the joint decision.

By inducing a tree structure on the correlations of the users' signals,the complexity of the MUD can be greatly diminished, as described by R.E. Learned in a PhD Thesis, MIT, January 1997, “Low Complexity OptimalJoint Detection for Over-Saturated Multiple Access Communications.” Seealso R. E. Learned, A. S. Willsky, and D. M. Boroson, “Low ComplexityOptimal Joint Detection for Over-Saturated Multiple AccessCommunications,” IEEE Trans. On Signal Processing, Vol. 45, No. 1,January 1997. Both of these references are incorporated by reference intheir entirety. A tree structure guarantees that users will havecorrelation with only a subset of other users. If a user has correlationwith another user then these users must be related through a path in thetree. Users that do not lie on the same path through the tree areorthogonal. This packing greatly reduces the computational complexity ofthe MUD by exploiting the relationship between the signals.

As described herein, one embodiment of the present invention is a methodfor combining multicarrier systems with tree structured signal packing.Using this structure, multiple-access systems can benefit from thefrequency diversity and simple signal parameter estimation provided bymulticarrier systems in addition to the ability to pack more users in agiven bandwidth and detect and decode all interfering users symbolstreams as provided by MUD. The advantages of such a system are evidentand a case for the combination of MUD and multicarrier systems was madeby X. Cai, S. Zhou and G. B. Giannakis in “Group-orthogonal MulticarrierCDMA”, IEEE Trans. Commun., vol. 52, pp 90-99, January 2004. However,this reference does not provide a general method for designingmulticarrier systems to simplify the MUD and it further assumes thatthere are only as many users as available dimensions. The presentinvention provides for more users than dimensions.

One embodiment of a signal packing scheme on the present inventionenables the system to maintain more users than dimensions while makingthe MUD in the receiver simple enough to implement in real time.Moreover, the narrow band flat fading quality of multicarrier signalingallows for a simple multiuser parameter estimation procedure.

In addition to having benefits for RF multicarrier systems, treestructuring could also be useful in an optical system that useswavelength division multiple access (WDMA), which is identical inconstruction to RF multicarrier MA systems. In this case opticaldetectors and minor modifications to the MUD are used to facilitate thephysical limitations in detecting optical signals.

Tree Structured Packing to Achieve Overloading

In a well-designed underloaded system, all the users ideally employmutually orthogonal signaling waveforms (ie. signatures), or at leastnearly uncorrelated signatures. Extending this working definition, afully loaded system accommodates the maximum number of users possiblewhile still satisfying the orthogonality or correlation criterion. Ameans for overloading a multiple access systems to allow for thereliable separation/detection of more users than available dimensions isknown to those in the art.

In general, the overloaded system contains at least some users whosesignatures are not orthogonal, or who have sizeable correlations.However, by proper structuring, one may design a system to optimallyaccommodate this overloaded condition. The user signatures (synchronousor asynchronous in time) in Learned's construction of an overloadedsystem are constructed to obey a tree-like interference rule. Moreover,the tree-structured signals in Ross and Taylor's construction in thepaper J. A. F. Ross and D. P. Taylor, “Multiuser signaling in thesymbol-synchronous AWGN channel”, IEEE Transaction on InformationTheory, 41, July, 1995, are in some sense an optimal way to create atree relationship among the various interfering users. Most importantly,however, is the fact that any set of tree structured signals will likelyresult in a robust multicarrier interference multiple access system, soan optimal choice of the signature set and tree is only one embodimentof the present invention.

One realization of a process for creating the user signature waveformsto obey a tree-structure of cross-correlation is to proceed in atree-like fashion, such that 1) the leaf nodes all correspond tomutually orthogonal signatures, and 2) the connecting nodes thencorrespond to signatures formed from linear combinations of theirchildren or, more generally, have non-zero or non-negligible innerproducts with signatures corresponding to their children, but have zero(or nearly zero) inner products with the other non-child leaf nodes.

According to Learned's example for creating a set of tree-structuredsignatures, in general, there are four requirements:

-   -   (a) The specification of the tree structure (i.e. the        connections among the nodes).    -   (b) The specification of a set of orthogonal basis functions to        assign to the N leaf nodes that terminate the tree.    -   (c) The specification of weights to use in the linear        combinations of lower level nodes that form the upper level leaf        nodes.    -   (d) Optionally, the ability to delete signatures at any node        when specifying the system.

Thus, Learned's scheme is limited to the above realization of a processto create a set of signatures that obey tree-structured crosscorrelations and requires exact mathematical combinations of lower nodesignatures to create upper node signatures. The present inventionencompasses the more general method for which a set of tree structuredsignatures can be created to include a more adhoc process that does notrequire exact mathematical combinations of lower node signatures tocreate upper node signatures, but instead, creates a parent signature tobe correlated with it's children signatures. Thus, with respect to thepresent invention, it is not required for that parent signature tostrictly be a linear combination of the child signatures. This moregeneral extension allows for building a tree-set of signatures in theasynchronous case, as well as in the case for which specific detailedknowledge of any transmitter or channel distortions are unavailable, orare not required to correctly model.

The reduced complexity optimal tree-MUD algorithm can be applied as longas cross correlations among any set of received user signatures across alevel of the tree are orthogonal (or nearly so) while any set ofreceived user signatures that lie along a connected path of the treehave a non-zero cross correlation. A portion of the invention thataddresses building tree-structured sets of OFDMA and MC-CDMA are thusgeneral in the actual procedure that one would utilize.

Tree-Packing Within the Multicarrier Framework

An assumption to the general processing is that every user in the systemis equipped with the capability for multicarrier communication over agiven bandwidth. The bandwidth is divided into subintervals calledsubchannels or subcarriers. The number of subcarriers is a function ofthe total multicarrier symbol length and the amount of bandwidth that isavailable. A further assumption is that the symbol time on eachsubcarrier is long enough to guarantee flat fading on each subcarrier.This is determined by the coherence bandwidth or delay spread of thechannel. Every user will be assigned a subset (not necessarily a propersubset) of the set of subcarriers. The user can utilize the subcarriersin a number of different ways, depending on the network architecture.

Each user may send one symbol on each subcarrier, the same symbol onevery subcarrier, or some trade off in between based on a precodingmatrix. Sending the same symbol on every carrier can be understood as aspecific simple type of precoding, namely repetition precoding of thesymbols for assignment to the subcarrier set. Such a system is generallydescribed by Z. Wang and G. B. Giannakis, “Wireless MulticarrierCommunications Where Fourier Meets Shannon,” I.E.E.E. Signal ProcessingMagazine, Vol 17, No. 3 (May, 2000), pp. 29-48.

It is well known by those proficient in the art of multicarriercommunications that it is advantageous for users to send everyinformation symbol over many carriers to decrease the probability that asymbol is “lost” by being sent over a subcarrier that is severelyattenuated due to fading. This can be accomplished in many ways fordifferent types of systems. For example, OFDMA systems will use errorcontrol coding across the subcarriers to diversify the informationthrough the channel. In general, however, the amount and type ofprecoding should be determined based on the amount of frequencydiversity that is required and the nature of the communication link.

Adding the desire to pack signals in such a way as to maintain a treestructure at the receiver so that the efficient Tree-MUD can be used,adds another consideration for the choice of preceding. For example, alimited amount of precoding may be desirable for maintaining the treestructure. The trade off between precoding and strict adherence to thetree structure should be done on a case by case basis, depending on thechannel, whether it is an uplink or downlink system, and thecomputational complexity allowed at the receiver. For the remainder, itshould be understood that various techniques can be used to attainfrequency diversity and the tree-packed multicarrier framework is ageneral concept since the tree packing can be applied in severaldifferent situations.

In an uplink scenario every user's signal will be subjected to adifferent channel due to the spatial separation that is assumed, so thedesign considers the worst case coherence bandwidth. In a downlinkscenario, the channel that every user's signal goes through is the samefrom a single receiver's point of view. Different users have a differentchannel from themselves to the base station but since the base stationis transmitting all the users' signals from the same location, they allgo through the same channel when arriving at any given receiver. Thesetwo different scenarios, the uplink and the downlink, can both benefitfrom the tree structured multicarrier approach described herein.

Referring to FIG. 3 a diagrammatic perspective of a broadcast channelembodiment is depicted. The broadcast tree structured multicarriertransmitter satellite 300 communicates in a bi-directional fashion witha number of tree MUD receivers 310, 320, 330, and 340. The satellite 300can control each of the users' signature waveforms to adhere to the treestructure and provide the necessary information to each terminal, andwherein each terminal can then perform MUD on its branch of the tree. Ina further embodiment, the satellite can also perform the tree-MUD toextract the signals from all the transmitting terminals, if they havebeen assigned a tree-OFDMA set of interfering signals.

There are several ways in which the tree structure can be applied to amulticarrier system. The main modes of operation generally depend on thenetwork assumptions. For example, in an overloaded downlink scenario amethod of imposing the tree structure might be using MC-CDMA. In thiscase a subcarrier assignment can be utilized as described in the art.The base nodes of the tree structure are formed by selecting groups ofsubcarriers and assigning the same number of users as subcarriers toeach group. Then orthogonal spreading codes can be used to separate thebase users in the same group. The number of base nodes will be equal tothe total number of subcarriers. Note that the base set of signaturesignals constructed this way comprises a typical set of orthogonalMC-CDMA users.

System Architecture

FIG. 4 is a block diagram of a tree-structured multicarrier transceiverconfigured in accordance with one embodiment of the present invention.For purposes of discussion, assume that a wireless communication networkincludes a number of such transceivers and a number of users at any givetime and operable in several systems such as MC-CDMA and OFDMA.

As can be seen, the transceiver 400 includes any other user environmentestimator module 405, a sub-carrier code selector 410, a multicarriersignal transmitter 415, a low complexity multiuser detector (MUD) thattakes advantage of the tree-structured cross correlations and errorcorrection decoder module 420, and a stream of interest selector 425.The received signal is a composite of many interfering signals, some ofwhich are from the same wireless communication system in whichtransceiver 400 exists, and other of the interfering signals may be fromneighboring communication systems.

On the receiver side, the received signal is received by the transceiver400 and processed through the receiver components, which include the lowcomplexity “tree” MUD and error correction decoder module 420 and thestream of interest selector 425. The signal of interest is output, andcan then be provided to its intended destination. On the transmitterside, information relevant to the other user environment is assessed bythe estimator module 405, and provided to the transmitter components,which include the sub-carrier code selector 410 and the multicarriersignal transmitter 415. A multicarrier transmit signal that isstructured based on the other user environment information is thenwirelessly transmitted by the transmitter.

The environment estimator module 405 is configured forestimating/tracking the number of interfering signals, and can beimplemented with conventional technology. However, variations will beapparent in light of this disclosure. For example, the estimator module405 can be configured as described in the commonly assigned U.S. patentapplication Ser. No. 10/228,787, titled, “Parameter Estimator for aMultiuser Detection Receiver”, which is herein incorporated by referencein its entirety. The MUD-required set of parameters associated with eachinterfering signal is part of the MUD receiver processing provided bymodule 420 of the transceiver 400.

The sub-carrier code selector 410 can be configured to implement any oneof numerous optimization techniques for facilitating the code selectionprocess. In one particular embodiment, the environment informationoutput by the estimator module 405 is provided to the code selector 410.The code selector 410 then determines an optimal sub-carrier coderepresented by the environment, and searches a look-up table to identifyan actual code that is closest to the optimal code detected by theselector 410. How the code selector 410 determines an optimalsub-carrier code represented by the environment is discussed in moredetail herein.

The code selector 410 can be implemented, for example, as a set ofinstructions executing on a digital signal processor or other suitableprocessing environment that can be configured to carry out the codeselection functionality. Note that other modules of the transceiver 400,such as the estimator module 405, transmitter 415, MUD and decodermodule 420, and the stream of interest selector 425, can be implementedin whole or part in the same processing environment. Alternatively, eachmodule can be implemented in its own dedicated processing environment.Numerous configurations are possible here, and the present invention isnot intended to be limited to any one such embodiment.

The multicarrier signal transmitter 415 can be implemented withconventional technology, and transmits a multicarrier signal inaccordance with the multicarrier code selected by the code selector 410.Note that the transceiver 400 may further include other circuitry orcomponents not shown in FIG. 4, such as an analog front end thatconverts the coded signal to its analog equivalent, amplifies thatsignal, and then transmits via an antenna. Various filtering is possibleas is known in the art to condition signals.

On the receiver side, the low complexity MUD and error correctiondecoder module 420 receives the composite signal (including a targetsignal and one or more interfering signal). Note that tree structuringof the interference (as provided by the transmitters at other nodes inthe network) will allow for low complexity implementations of very highquality parameter estimation and MUD algorithms. In one particularembodiment, the exponential complexity in the number of ancestors in thetree is no more that 3 or 4. This processing results in a very lowcomplexity on a par with minimum mean squared error (MMSE) MUD modules,for example. Note, however, that low or otherwise reduced complexity MUDalgorithms are not required, and other embodiments of the presentinvention may employ full-complexity algorithms (e.g., based on maximumlikelihood principle).

In one particular embodiment, the MUD portion of module 420 isconfigured with a tree joint detection algorithm that is described in“Low Complexity Signal Processing and Optimal Joint Detection forOver-Saturated Multiple Access Communications”, R. E. Learned, A. S.Willsky, and D. M. Boroson. Other tree joint MUD algorithms can be usedhere as well, as will be apparent in light of this disclosure. In anycase, the MUD and error correction decoder module 420 is configured toproduce a bit stream for each interfering signal included in thereceived composite signal. The stream of interest selector 425 isconfigured to select the stream/signal of interest, and can beimplemented with conventional technology.

Prior to discussing the tree joint detection algorithm, note thattime-bandwidth restrictions on any communication system limit thedimension, N, of the space of possible user waveforms. Adopting thecommonly-used vector space framework, the N-dimensional signal space canbe identified with IR^(N) and the multiuser joint detection problem canbe stated as follows: for a given set of user waveforms represented insignal space by the set of signal vectors, {s_(k)}₁ ^(K), s_(k)εIR^(N),the general uncoded detection problem is to compute an estimate ofweights, b, from an observation rε

^(N):

$\begin{matrix}{r = {{{\sum\limits_{k = 1}^{K}{b_{k}s_{k}}} + {\sigma\; n}} = {{Sb} + {\sigma\;{n.}}}}} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$

Here, K is the number of users, and bε{[b₁ . . . b_(K)]^(T)|b_(i)εP_(i)}where P_(i) is some finite set of real amplitudes and b_(i) is andindependent and identically distributed sequence. For Pi having Melements, this is M-ary PAM. S=[s₁, . . . , s_(K)] is an N×K matrixwhose columns are user signal vectors as seen at the receiver, and n isa real Gaussian vector of mean zero and identity covariance, and σ isthe noise standard deviation.

Tree Cross-Correlation Structure

An example tree structure is shown in FIG. 5 a. The connections describethe correlation properties of the nodes involved in the system. A nodeis correlated with another node if there is a path to a higher node(ancestor) or lower node (descendent). Nodes at the same level aredesigned to be orthogonal, or nearly so. The tree shows the correlationstructure needed among signature vectors within a signature vector setto allow for the low complexity tree-MUD to result in high qualitydemodulated symbol streams for all the interfering users.

The advantage of having the tree structure is that the maximumlikelihood MUD can be constructed in a recursive manner. This is becausethe decisions about each users' symbols are only functions of theancestors and decedents of that node on the tree. A multicarrier systemcan be designed to support the tree structure in an overloadedcommunication environment, and a general multicarrier-multiusercommunication system is described herein and adapted to this framework.

The geometric structure imposed on a signal vector set is best describedby saying that the set of signatures has tree-structuredcross-correlations. Specifically, S will have the desired structure ifthe signal vectors can be assigned to the nodes of a tree. The treepictorially conveys the following required relationships among usersignal vectors: each vector at a given level of the tree is orthogonalto all other vectors at that level; and a signal vector is correlatedonly with its ancestor vectors (parent, grandparent, etc.) and itsdescendent vectors (children, grandchildren, etc.). Both linearlydependent and linearly independent sets of signature vectors may becreated to have tree-structured cross-correlations.

In one embodiment, the MUD module 420 is adapted to find the optimalsolution for both cases. The constraint of tree-structuredcross-correlations, while very particular, actually allows aconsiderable amount of flexibility in designing user waveforms. Given atree, a signal set can be constructed that possesses the desiredcross-correlation structure. Assume, for example, that waveforms at thebottom level of the tree comprise an orthogonal set. An orthogonal setcan be obtained at any level (i.e., the 1^(th) level) by constructing asignal at each node at this level as a linear combination of the signalsat its bottom-most descendent nodes. Since a tree with orthogonalsignals is assigned to the lowest level nodes, the sets of descendentsfor distinct nodes at the 1^(th) level are disjoint, and consequentlythe signals created at level 1 are mutually orthogonal.

It follows that one general construction of a signal set withtree-structured cross-correlations requires: (a) the specification ofthe tree structure (e.g., including the number of levels, L, and theparent-child relations for all levels of the tree); (b) thespecification of any orthogonal basis s₁, s₂, . . . s_(N), of IR^(N),which is then assigned to the N nodes on the finest scale of the tree;(c) the specification of the weights for each of the linear combinationsused to construct signals from their bottom-level descendents; andpossibly (d) the deletion of signals at any of the nodes. Thisformulation allows for considerable flexibility in designing the signalset since any choices that satisfy (a) through (d) will lead to thedesired geometric structure on the signal set. Note also that (d)provides the flexibility to capture linearly independent sets with thedesired correlation.

As detailed herein, another more general approach to creating a set oftree-structured signatures allows for unknown transmitter or channeldistortions and asynchronous reception of signals.

Tree Joint Detection Algorithm

The optimum joint detector, which is known in the art, for the problemstated in Equation 1 chooses the weight vector estimate, {circumflexover (b)}, according to the nearest neighbor or minimum distance rule.

$\begin{matrix}{\hat{b} = {\arg\;{\min\limits_{b \in P^{K}}{{{r - {Sb}}}^{2}.}}}} & \left( {{Equation}\mspace{14mu} 2} \right)\end{matrix}$

For ease of discussion, assume each user employs the same M-ary PAM,where b_(i)εP, ∀i. Note, however, that this assumption is not essentialto the operation of the tree algorithm or to all embodiments of theinvention. A multiple access system employing an arbitrary set of signalvectors, S, can achieve the optimal detection of the above detectorthrough an exhaustive search. In one particular embodiment, the detectorperforms M^(K)−1 comparisons to find the best estimate.

If the signal set has been constructed to have the treecross-correlation structure previously described, the optimum detectorof Equation 2 can be achieved through a tree-structured algorithm thatoffers a huge reduction in the number of comparisons, as detailed inLearned's thesis. In particular, because of this tree structure, asignature at a given node is correlated with all signatures at itsancestor and descendent nodes and is orthogonal to all other signatureson the tree. As such, the weight estimate, {circumflex over (b)}_(n), ata given node, n, will effect the estimates at descendent and ancestornodes but will not effect the other estimates on the tree.

Consider, for example, the example tree structure in FIG. 5 b. Note thechoice of the weight estimates for users 1 through 4 having signalvectors s₁, s₂, s₃, and s₄. These vectors are mutually orthogonal, andare also orthogonal to s₅, s₆, s₇, and s₈, and s₁₀ but not to s₉ ands₁₁. Since s₅ through s₈ and s₁₀ are also correlated with s₁₁, thedecisions on weight estimates for these users are not decoupled fromthose for s₁ through s₄. However, the decisions on weight estimates forthese users can be decoupled by looking, instead, at the conditionalestimates.

More specifically, for each possible pair of weight estimates for s₉ ands₁₁, the optimal weight estimates can be computed for s₁ through S₄independently (i.e., the problem to be solved for each of these weightestimates is decoupled not only from the other three but also from theweight estimates corresponding to s₅ through s₈ and s₁₀). The result ofthis calculation for s₁ through s₄ can be thought of as producing aconditional weight estimate table (i.e., for each possible pair ofchoices for the weight estimates for s₉ and s₁₁, the optimal weightestimates are known for s₁ through s₄). Similarly, for each pair ofpossible weight estimates for s₁₀ and s₁₁, the optimal estimates for s₅through s₈ can be computed.

If s₉ is now considered, this process can be iterated: for each possiblechoice of weight value for its ancestor s₁₁ and with knowledge of thejust-constructed conditional estimate table for its descendents s₁through s₄, the optimal estimate can be computed for s₉ in a mannerdecoupled from the analogous computation for s₁₀. This gives conditionalestimate tables for s₉ and s₁₀ that can be used to determine the optimalestimate for s₁₁ at the top of the tree. Conceptually, once thisestimate is determined, it is a simple matter of successive tablelook-ups, with propagation down the tree continuing to determine theoptimal estimates first for s₉ and s₁₀ and then for their descendents.

As this example illustrates, the tree detection algorithm takesadvantage of the tree structure and sweeps through the tree from bottomto top, creating a conditional weight estimate table at each node. Thetable of decisions at a given node is conditioned on weight decisions ofthe ancestors and is a function of weight decisions of the descendents.Since each conditional estimate table requires entries for each possiblecombination of weights at all ancestor nodes, the number of computationsneeded to create a table and the size of the table is exponential in thenumber of ancestors (since if there are 1 ancestors there are M^(l)possible sets of weight values for these ancestors). This complexitydecreases exponentially as 1 decreases (i.e., as the algorithm movesfrom the bottom to the top of the tree, the number of decisions made ateach level decreases exponentially until there is only one decisionassociated with the top node of the tree). The full weight vectorestimate for all user weights is a by-product of the last decision atthe top of the tree.

While the complexity of the procedure is exponential in the number oflevels in the tree (which bounds the number of ancestors of each node),note that the actual algorithm complexity is extremely modest. If thetree were of uniform construction (i.e., if there are Q childrenemanating from each node, the number of levels of the tree islogarithmic in the number of users, K. Hence, the overall complexity is,then, bounded by a very low-order polynomial in K.

Overloaded MC-CDMA via Tree-Structured Signature Sets

Referring to FIGS. 6 a-6 d, the depicted system shows 16 subcarriers andvarious use of those 16 subcarriers assigned to 16 orthogonal users inan MC-CDMA system. Each subplot shows four users occupying a total of 16subcarriers, and each user's carriers are designated as follows:

b Users ;

r Users;

k Users;

g Users

This figure is labeled with the same coding used in the tree structureshown in FIG. 10, and will be built upon to create a tree of interferingsignature signals for use in an overloaded tree-structured MC-CDMA. Fouruser signals are shown in each plot as is indicated by the use ofdifferent types of lines. For example, in FIG. 6 a, there is a singleuser that transmits its symbol on four carriers with the four frequencypulse carrier signals as shown, each one shown as a “positive-pulse” infrequency. In FIGS. 6 a-d, one user from each group is shown per plot,where there are four users from the dotted-line group, one in each ofthe FIGS. 6 a, b, c, and d; there are also four from the dashed linegroup, one in each figure, etc.

The first subplot, FIG. 6 a, shows nodes b1, r1, k1, and g1. The nextsubplot, FIG. 6 b, shows users b2, r2, k2, and g2 and so forth. For thefour users in group b, each user is assigned a 4-subcarrier signaturesignal using the same four subcarriers, where each user's signal ingroup b is orthogonal to the other three users' signals in group bbecause of their spreading codes. Moreover, the group b users' signaturesignals are orthogonal to all other users' signature signals (all othergroups) because group b users employ a different set of subcarriers thanany of the other groups (r, k, and g). In this example, the users ingroup b can be assigned Hadamard codes to make them separable. The useof the Hadamard code is for illustrative purposes only and not to bedeemed as limiting. An identical explanation as that above for group busers is true for each of the other groups, r, k, and g.

Referring again to FIGS. 6 a-6 d, the 16 signals form the base nodes ofthe tree structure in a tree-packed MC-CDMA system. These 16 signaturesignals could, coincidentally, be used in a fully loaded state of theart MC-CDMA system. Each figure shows a single user's carrier set foreach of the four groups. Specifically, the top plot, FIG. 6 a, showsusers b1, r1, k1, and g1, where each of these four users employs asignature signal that is comprised of four subcarriers. The second plot,FIG. 6 b, shows users b2, r2, k2, and g2, each using the same foursubcarriers as its corresponding group user's signature signal from FIG.6 a. Note that the user signatures of the same group defined in theplots are orthogonal due to a careful weighting of the subcarrier pulses(this is code orthogonality). Users corresponding to different groups ineach plot are orthogonal due to the fact that their signature signalscomprise disjoint sets of subcarriers.

The complete set of interfering overloaded user signature signals arethen created from these 16 orthogonal users. See FIG. 10 for an exampleof a complete tree, where these 16 signature signals would correspond tothe bottom 16 nodes of this tree. Beginning with the 16 signatures thatcomprise a fully loaded MC-CDMA system as shown in FIG. 6, we might adda 5^(th) user to each subcarrier grouping. As shown herein, the user'ssignature signals will correspond to a node in the tree directly abovethe base node groups. The 5^(th) user, as illustrated by this example,would use any combination of the subcarriers used by the users of thesame group. For example FIG. 7 shows a viable subcarrier assignment foreach of 4 new users (one in each group), b5, r5, k5, and g5. Each newuser is correlated with all base node users of the same group and isorthogonal to all signature signals corresponding to the base nodes ofother groups.

Referring to FIG. 7, a 5th user is added to each group to make thesystem overloaded. This figure shows four signature signals, one foreach group. These four signature signals are orthogonal to one anotherbecause they use disjoint sets of subcarriers. Each signal, however, iscorrelated with the signals of the corresponding group in the base setof signals shown in FIG. 6.

Combining FIGS. 6 a-6 d and FIG. 7 into FIG. 10 offers an illustrationof the remainder of the tree building process, and, in this example, isformed from combinations of the subcarriers associated with the basenodes in that group. An example realization of the signature signalsthat correspond to the next level up in the tree is show in FIG. 8 wheresignature signals for users c1 and n1 are created. The signature signalfor user c1 is correlated with all signatures that lie in a direct pathbeneath it on the tree, and it is orthogonal to all other signaturesignals corresponding to the other nodes in the tree.

Referring to FIG. 8, this figure shows two user signature signals, oneper group, corresponding to users c1 and n1 in the tree. Each usersignature signal is built from 8 subcarriers each. As detailed herein,there is a link between these signature signals and their correlatedsets upon which they were built. Note that to maintain tree structure,in this example, the 4 orthogonal subcarrier sets used in the definitionof the 4 user groups in FIGS. 6 a-6 d have been split into 2 disjointsets of 8 subcarriers each, or 2 sets comprised of two groups each,assigning a single group to no more than one set.

The tree is completed with the top node user, p1, having the signaturesignal shown in FIG. 9. The signature signal for user p1 is correlatedwith all other user signatures corresponding to all other nodes in thetree.

Referring to FIG. 9 and FIG. 10, the top node in the MC-CDMA example isillustrated. This user is correlated with all other users in the systemand uses all the subcarriers. Some nodes are replaced by their spectralrealizations to further detail the assignment of subcarriers to nodes.It should be understood that the first four base nodes on the lowestlevel use the same subcarriers, followed by the next four using adifferent subcarrier set in frequency, and so forth.

Notice that the tree structure used in this MC-CDMA example would becorrupted in an uplink scenario because each user's signal goes througha different channel. For example, if two b users go through differentchannels then their received spectrums are no longer orthogonal becausethe codes have been changed by the channel. However, the b users willstill be orthogonal to the r users because they use different carriers.To overcome the loss of orthogonality caused by the channel the signalscan be precoded to account for the channel so they arrive at thereceiver orthogonally. If precoding is not feasible then the tree asshown in FIG. 10 is not the right tree for use by the tree-structuredMUD and must be adapted to account for the correlation between thesebase nodes. Specifically for the channel corruption case just mentioned,the 5 b-users would appear in a straight line path beneath user c1 andthe 5 k-users would also appear in a straight line path beneath user c1.The same would be true for the r-users and g-users. The result wouldstill be a tree, but with 7 levels instead of 4, upon which the tree-MUDwould still operate efficiently and optimally. The computationalcomplexity, however, would be larger than for the case of the tree shownin the figure (the tree with 4 levels.)

Overloaded OFDMA Via Tree-Structured Signature Sets

Another possible realization of the tree structured signal packing tomulticarrier communications is to use OFDMA on an uplink scenario. Inthis case the base nodes of the tree are formed from each subcarrier. Sothe base nodes do not necessarily correspond to a user's signal, but,more generally, to a subcarrier in the bandwidth. This means that asingle user could correspond to a single node at the base of tree (ifthis user requires only a single subcarrier, possibly due to having areliable single carrier channel) or to multiple nodes at the base of thetree (the usual OFDMA realization to overcome independent fading amongthe subcarrier channels.) The reason for structuring the tree this wayis that each information symbol to be detected in the MUD is treated asa “virtual user”.

Referring to FIG. 11, a block diagram of a wireless multicarrier systemwith a tree structured MUD is shown. In this particular embodiment,there is a multicarrier transmitter 1100 coupled to a signature module1105 for User 1. The transmitted signal is transmitted from some form oftransmitter antenna 1110. There is typically a plurality of suchtransmitter sections 1150 for each of the K users. The transmittedmulticarrier signals are received by a multicarrier MUD receiver antenna1120 for subsequent processing in the MUD receiver 1130. Each user, forexample, could be an OFDMA transmitter communicating with a base stationin a cellular system.

As long as the number of independent streams coming from all the usersis not greater than the number of subcarriers the system is notoverloaded, and it will have a traditional OFDMA system. If the systemis overloaded, the processing can be accomplished in a tree structuredway as detailed herein. Overloading an OFDMA system to adhere to thetree-structure recommendation may be done is several ways. One exampleis offered herein, however the present invention is not limited to aparticular implementation.

As an illustrative example to creating the tree-structured signature setfor overloaded OFDMA operation see FIG. 12. The bottom nodes, for thisexample, represent individual subcarriers. A bottom node user mighttransmit it's symbol on more than one subcarrier, as indicated here withthe letter portion of the node labels, so that the “b” user has beenassigned the “b” subcarrier set, the nodes marked b1, b2, b3, . . . b7.To preserve tree structure so that we obtain the benefit of the lowcomplexity tree-MUD, an information symbol from any single bottom-nodeuser is “connected” to any other information symbol from that same userbased on the way that user has precoded its information symbols over thesubcarriers assigned to that user. An attractive benefit to using thismethod of ignoring the “connections” among the subcarriers assigned to asingle user, is that the tree structure is maintained even if thechannels of individual users widely vary. All of the higher level userssend the same symbols over the set of multiple subcarriers as denoted bythe parent-child connections in the tree.

A Detailed Example Tree Structure for an OFDMA System.

One embodiment of an overloaded OFDMA example tree structure is shown inFIG. 12. Nodes are named in accordance with the transmitter they comefrom (i.e. the node b5 is the 5^(th) carrier transmitted by user b).There are a total of 7 users (b, r, k, g, c, y, and p) and 21subcarriers (the number of base nodes).

The base nodes are conventional OFDMA transmitters that send informationover several subcarriers. For this example, there are three users, b, r,and k. This set of three users comprise a typical OFDMA system whereeach user has been assigned its own set of subcarriers that is disjointto all other subcarriers in use by the other users. The higher levelusers have each been assigned a single signature signal comprised of acarrier subset, being a subset of the base subcarriers to which they are“connected” on the tree. Each signature signal corresponding to thesecond level of the tree is based upon a carrier subset that is disjointwith the carrier subsets used for the signature signals corresponding tothe other users at this same level of the tree. Each user at the secondlevel of the tree uses its entire set of subcarriers to send a singlesymbol, specifically, a user at this level will send a symbol overcombinations of its carriers. In this example, the y1 user will eitherrepeat, use a direct sequence code over, or precode its information overseveral subcarriers, where, as shown in the figure, it uses two of the ruser's subcarriers (r1, r3), two of the k user's subcarriers (k4, k7),and one of the b user's subcarriers (b6).

This particular approach to overloaded tree-structured OFDMA is nicebecause it can be adopted for multiple quality of service assignments.For instance, the base node users may have better signal quality at thereceiver so they can use separate information symbols on each carrierwhereas, the higher level users may have lower signal quality andrequire lower quality of service so preceding a symbol over multiplecarriers could be useful for gaining diversity and maintaining the userslink at a lower data rate.

Note that in all cases above, the channel can be accounted for and thetree structure is maintained at the receiver even though each signalwill go through a different channel. Note that even if strict treestructure is not maintained, the tree-MUD can still be used as a lowcomplexity suboptimal MUD instead of a low complexity optimal MUD. Morespecifically, if the cross correlations across a level of the tree aresmall relative to those for received signals corresponding to alineage-path in the tree, then the tree-MUD can still be implementedwith only a negligible loss as compared to the case where strict treestructure was maintained.

Methodology

FIG. 13 illustrates a method for building a set of spreading codes fortree-structured MC-CDMA communication in accordance with one embodimentof the present invention. The method can be carried out, for example, bya communications system configured with tree-structured MC-CDMAtransceivers configured as described in reference to FIG. 13. In onespecific embodiment, the method is carried out by the MC-CDMA codeselector 1310 of FIG. 13. It should be readily apparent that a similarprocess exists for OFDMA which is further described herein.

The method begins with identifying 1305 a set of “base” spreading codes.This set can be identified, for example, based on a priori knowledge ofthe communication system, or by listening to the system (e.g., usingparameter estimation techniques) and collecting the spreading codeinformation in real-time.

The method proceeds with building 1310 another set of spreading codes,called the parent spreading codes. In one embodiment, this new set oforthogonal codes is built by arbitrarily making “sibling groups” of oneor more base spreading codes per group, not allowing any one basespreading code to appear in more than one group, and then linearlycombining all the base spreading codes within a sibling group. Noticethat in the degenerate case where there is only one code in a siblinggroup, the base spreading codes are re-assigned, which is an acceptableoutcome and often desirable when a near-far scenario naturally exists inthe field.

The method proceeds with building 1315 another set of spreading codes,called the grandparent spreading codes. This set of codes can be built,for example, in the same manner as the parent spreading codes are built,but with the parent spreading codes acting as the basis set. Thus, theset of grandparent spreading codes is built by arbitrarily makingsibling groups of one or more spreading parent codes per group, notallowing any one parent spreading code to appear in more than one group,and then linearly combining all the parent spreading codes within aparent sibling group.

FIG. 13 shows the process if the linear combination rule is to be usedto create signature signals from other signature signals. A similarprocess may be used, but, more generally, at each step, the newsignature signal is created from the set of all subcarriers used by thegroup of signature signals connected to as children or the group ofsubcarriers used by any of the descendants, including the correspondingbottom nodes to the current node under construction. This more generalprocess is specific to the example shown in FIG. 10. There could, in amore general case, be hundreds of bottom nodes, and possibly hundreds ofbase MC-CDMA users at the bottom of a tree, leading to many more levelsthan shown in this example, however, the process would simply continuein the same manner as put forth here, and would end when there are nomore nodes to combine. The process, in a more general form, alsoincludes a step of deleting any nodes on the tree after the creation ofthe full tree.

For an adhoc implementation, each user can be pre-assigned a dictionaryof possible spreading codes from which to choose, so that a givenpercentage of code corresponds to each level of the family tree. Forinstance, a radio could be assigned ten different codes from a threelayer tree: five codes from the sibling level, three codes from theparent level, and two codes from the grandparent level. Further, notethat radios can cycle through their code dictionary according to ametric, such as a metric based upon a component in a global positioningsatellite (GPS) signal or by a “reprogram” signal.

In addition, each radio and/or message can be assigned a priority valuethat can be used to help keep the MC-CDMA communication system fromfailing in times of extreme demand or in times of reduced channelresources. In operation, a radio that wishes to transmit will performthe parameter estimation to determine the loading of the system and thecodes currently in use. The radio will decide if it can begin totransmit (or continue to transmit). In one such embodiment, the radiowill only transmit if a look-up or metric calculated from the currentsystem loading and the priority value of the radio or message allows foran “ok” to transmit.

In one embodiment, a radio will choose which of it's spreading codes touse for transmission by, for example, first determining the receivedpowers and rates of each active user (which is low complexity because ofthe tree-structure), and then determining the approximate distance alongwith a rough channel degradation value for its own communication linkwith its own destination receiver with the list of users, each of theircodes, powers and rates. A low complexity ad-hoc near-optimization canthen be computed to determine the first code to try (should often be thefinal code that will work for the duration of the link).

Note that a spreading code already in use may be employed. This wouldhappen if the currently active user, transmitter A, has an estimatedreceived power at receiver B that is in excess of the data rate of thisuser. This means that receiver B can receive both the signal fromtransmitter A and the signal from transmitter B perfectly, even iftransmitter A is behaving like the traditional spoofer to thetransmitter/receiver B. Receiver A that is the intended destination forthe signal from transmitter A will either be able to receive the signalfrom transmitter A without degradation, most of the time not even havingto account for the presence of the signal from transmitter B.

The receiver will pay attention to a sub-set of bit streams included inall the bit streams provided by the MUD module. In particular, thereceiver will pay attention to a sub-set of bit streams that correspondto the dictionary of codes that it knows the radio to use. From thosecodes, the receiver will know which is the right one when it finds atarget stream in a header or the like. Alternatively, the codedictionary can be eliminated by configuring or otherwise forcing eachreceiver to look at a header or other such portion of each signal's datauntil the receiver finds the one it is looking for. Either way, the bestbit error rate (BER) will be achieved when all signals are jointlydecoded so this information is available.

Note that if the receiver of the radio determines the link to be poor orthe packet to be in error, that radio can transmit back to thetransmitter and advise the transmitter to either choose another code(e.g., which can be a short message) or use a code that the receiver haschosen (e.g., from performing an optimization algorithm using a look-uptable).

In the case where no feedback from receiver to transmitter is available,there are alternative ways for communicating through possible bad links.For example, one way for communicating through a possible bad link is tochange the code used at predetermined intervals. Note that usingpredetermined intervals will make the receiver's job easier. Another wayfor communicating through a possible bad link is to change the code atrandomly determined intervals. Here, since the receiver is alwaystracking the number of active signals and all their parameters, it willfollow the intended transmitter's switch, but will have to re-examineheaders to regain lock on the transmitter of interest. In addition,special care must be taken to maintain the tree structure when assigninga new code.

The foregoing description of the embodiments of the invention has beenpresented for the purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed. Many modifications and variations are possible in light ofthis disclosure. It is intended that the scope of the invention belimited not by this detailed description, but rather by the claimsappended hereto.

1. A tree-structured multicarrier interference multiple access (MC-IMA)transceiver for use in a multiple access environment having a number ofinterfering signals, comprising: an environment estimator moduleconfigured to estimate and track the number of interfering signals ofthe environment; a multicarrier signature signal selector configured todetermine a multicarrier signature signal as a function of theenvironment, and to identify an actual multicarrier signature signalthat is closest to the multicarrier signature signal whereintree-structured signatures are created without exact mathematicalcombinations; a signal transmitter configured to transmit a multicarriermultiple-access signal in accordance with the actual multicarriersignature signal identified by the multicarrier signature signalselector; and a multiuser detector (MUD) module configured to performtree structured multiuser detection signal processing, and to produce abit stream for each interfering received signal; wherein themulticarrier signature signal selector is further configured to build aset of parent multicarrier signature signals based on a known set ofbasis subcarriers used in the base set of multicarrier signaturesignals, and to build a set of grandparent multicarrier signaturesignals based on either the set of parent multicarrier signature signalsor the basis set of subcarriers from which those signals were comprised,and to continue creating ancestor nodes in the same manner until thereare no more nodes to combine in this manner, and wherein a code selectoris further configured to arbitrarily making sibling groups of one ormore base multicarrier signature signals per group, not allowing any onebase multicarrier signature signal to have a non-zero or non-negligiblecross correlation with any signal in more than one group, and creating anew multicarrier signature signal by constructing a new multicarriersignature signal from the basis set of subcarriers present in theaggregate of all the signals within a sibling group.
 2. The transceiverof claim 1 wherein said multicarrier signature signal selector isconfigured to determine an optimal multicarrier signature signal.
 3. Thetransceiver of claim 1 wherein said multicarrier signature signal is anymulticarrier signature signal that abides by the tree-structure.
 4. Thetransceiver of claim 1 further comprising a stream of interest selectorconfigured to select one or more bit streams of interest.
 5. Thetransceiver of claim 1 wherein the multicarrier signature signalselector is further configured to determine received powers and rates ofeach active user in the environment, and to determine a channeldegradation value for a communication link between the transceiver and adestination receiver.
 6. The transceiver of claim 1 further comprisingdeleting any of the nodes and corresponding signature signals from theset of tree-structured multicarrier signature signals.
 7. Thetransceiver of claim 1 wherein the code selector is further configuredto arbitrarily making sibling groups of one or more base multicarriersignature signals per group, not allowing any one base multicarriersignature signal to have a non-zero or non-negligible cross correlationwith any signal in more than one group, and creating a new multicarriersignature signal by linearly combining all the base spreading codeswithin a sibling group.
 8. The transceiver of claim 1 wherein the codeselector is further configured to arbitrarily making sibling groups ofone or more parent multicarrier signature signals per group, notallowing any one multicarrier signature signal to have a non-zero ornon-negligible cross correlation with multicarrier signature signals inmore than one group, and linearly combining all the multicarriersignature signals.
 9. The transceiver of claim 1 wherein the codeselector is further configured to arbitrarily making sibling groups ofone or more parent multicarrier signature signals per group, notallowing any one multicarrier signature signal to have a non-zero ornon-negligible cross correlation with multicarrier signature signals inmore than one group, and creating a new multicarrier signature signalfrom the set of subcarriers used within a sibling group.
 10. A methodfor performing tree-structured multicarrier interference multiple access(MC-IMA) communication in a communications system in a multiple accessenvironment having a number of interfering signals, said communicationssystem comprised of at least one transmitter for transmitting datasignals and at least one corresponding receiver, said method comprisingthe steps of: estimating and tracking the number of interfering signalsof the environment in said communications system; determining amulticarrier signature signal represented by the environment, andidentifying an actual multicarrier signature signal that is closest tothe multicarrier signature signal wherein tree-structured signatures arecreated without exact mathematical combinations; transmitting amulticarrier multiple access signal in accordance with the actualmulticarrier signature signal; and performing tree structured multiuserdetection signal processing, and producing a bit stream for eachinterfering signal; wherein determining a multicarrier signature signalcomprises: building a set of parent multicarrier signature signals basedon a known set of base multicarrier signature signals or the set ofsubcarriers used by the base set of users; and building a set ofgrandparent multicarrier signature signals based on the set of parentmulticarrier signature signals or the set of subcarriers used by thebase set of users; and continuing this progression upward by doing thesame for a set of great-grandparent multicarrier signature signals andso on, until all nodes have been linked, and wherein building a set ofparent multicarrier signature signals comprises: arbitrarily makingsibling groups of one or more base multicarrier signature signals pergroup or set of subcarriers used by that group, not allowing any onebase multicarrier signature signals to have a non-zero or non-negligiblecross correlation with signals in more than one group; and creating anew multicarrier signature signal by constructing a new multicarriersignature signal from the basis set of subcarriers present in theaggregate of all the signals within a sibling group.
 11. The method ofclaim 10 further comprising selecting any multicarrier signature signalthat abides by the tree-structure.
 12. The method of claim 10 furthercomprising determining an optimal multicarrier signature signal.
 13. Themethod of claim 12 wherein determining said optimal multicarriersignature signal further includes determining received powers and ratesof each active user in the environment, and determining a channeldegradation value for a communication link between the transceiver and adestination receiver.
 14. The method of claim 10 further comprisingselecting a bit stream of interest.
 15. The method of claim 10 furthercomprising deleting any of the nodes and corresponding signature signalsfrom the set of tree-structured multicarrier signature signals.
 16. Themethod of claim 10 wherein building a set of grandparent multicarriersignature signals comprises: arbitrarily making sibling groups of one ormore parent multicarrier signature signals or set of subcarriers used bya group of parents, not allowing any one parent multicarrier signaturesignals to have a non-zero non-negligible cross correlation with anymulticarrier signature signal in more than one group; and creating a newmulticarrier signature signal by linearly combining all the basespreading codes within a sibling group or constructing a newmulticarrier signature signal from the basis set of subcarriers presentin the aggregate of all the signals within a sibling group.